Method of improving stress corrosion resistance of aluminum alloys

ABSTRACT

METHOD OF HEAT TREATING ALUMINUM BASE ALLOYS OF THE ZINC-MAGNESIUM COPPER SERIES TO ATTAIN IMPROVED RESISTANCE TO STRESS CORROSION. THE METHOD COMPRISES QUENCHING THE ALLOY FOLLOWING SOLUTION TREATMENT IN A LIQUID HELD AT A TEMPERATURE OF BETWEEN ABOUT 230*F. AND ABOUT 260*F. FOLLOWED BY A TWO STEP AGING TREATMENT THE FIRST OF WHICH IS CONDUCTED AT A TEMPERATURE BETWEEN ABOUT 230*F. AND ABOUT 260*F. FOR PERIOD OF BETWEEN ABOUT 20 HOURS AND 30 HOURS AND THE SECOND AGING STEP IS CONDUCTED BETWEEN ABOUT 310*F. AND 340*F.

F. H. COCKS March 30, 1971 METHOD OF IMPROVING STRESS CORROSION RESISTANCE OF ALUMINUM ALLOYS Filed NOV. 21, 1968 h imommm 5 m2; @Z o mm O ON v (\I r (qaup amnbs 19d spunod 4o spuosnolu) 3 5: ommm mp (SQUWWUHH mm OJ. HWLL FRANKLIN H; COCKS INVENTOR.

BY 7% a ATTORNEY.

United States Patent 3,573,117 METHOD OF IMPROVING STRESS CORROSION RESISTANCE 0F ALUMINUM ALLOYS Franklin H. Cocks, Waltharn, Mass., assignor to Tyco Laboratories Inc., Waltham, Mass. Filed Nov. 21, 1968, Ser. No. 777,745 Int. Cl. C21d 1/50; C22f 1/04 US. Cl. 148159 7 Claims ABSTRACT OF THE DISCLOSURE The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 U.S.C. 2457).

This invention relates to heat treatable, high strength aluminum-base alloys of the 7000 series, and more particularly to a process for heat treating such alloys to substantially increase the time to failure when exposed to corrosive environments under conditions of high stress.

Although several theories have been advanced to explain the phenomenon of stress-corrosion cracking in aluminum alloys (i.e., failure caused by the simultaneous presence of stress and a corrosive environment), perhaps the most widely accepted theory correlates cracking with the simultaneous presence of stress and localized impoverished areas at grain boundaries. These impoverished areas are believed to be anodic in nature and are formed along the grain boundaries as a result of the diffusion of certain elements out of the lattice of the solid solution to form precipitate particles at these boundaries. When the alloy is exposed to a corroding medium, galvanic action occurs and results in the development of small cracks. It is further theorized that high local stresses accelerate oxide rupture and slip step emergence and thus accelerate the galvanic action. Continued corrosion of the anodic material increases the sizes of the initial cracks which, in turn, become sharpened by further concentration of the stresses. Failure of the alloy is typically sudden and dramatic.

For structural applications, such as may be found in the aircraft industry (e.g., wing and fuselage struts) where high resistance to corrosion is also necessary, weight penalties incurred by the necessity of increasing the size of the section to safely carry the required loads can be very significant. Hence, any process which will have the effect of decreasing the susceptibility of the alloy to stress corrosion cracking, without an accompanying loss of mechanical properties, has great application.

Accordingly, the primary object of the present invention is to improve the stress corrosion resistance of aluminum-base alloys while also retaining the mechanical properties at a high level.

Another object is to provide a method of heat-treating aluminum base alloys of the copper-magnesium-zinc series which is effective in markedly improving their resistance to stress-corrosion cracking.

A further object is to provide a method of heat-treating selected aluminum base alloys which can be applied in whole or in part to already fabricated members and which results in improved stress corrosion resistance.

A more specific object is to provide a process for increasing the capability of 700 series aluminum-base alloys (e.g., aluminum alloys of the zinc-magnesiumcopper series) to withstand the efiects of corrosion under conditions of stress, while maintaining the strength properties of such alloys at a level of at least of the high strength properties characteristic of the T6 or T651 temper. Related thereto is the additional object of providing an aluminum-base alloy of the copper-magnesiumzinc series which, when treated according to the process of the present invention, exhibits high resistance to failure under stress-corrosion conditions and retains at least 95 of the strength properties typical of such alloys in the T6 or T651 condition.

Described briefly, the present invention comprises subjecting the alloy to a solution heat-treatment, then to an oil quench, and thereafter to a two-phase aging treatment. The first phase of the aging treatment develops near full strength in the alloy; the second phase involves overaging and develops maximum resistance to failure under conditions of high stress and corrosion. The first phase involves more time but a lower temperature than the second phase.

Other objects and many of the attendant advantages of the present invention are described or rendered obvious by the following detailed description which is to be considered together with the accompanying drawings wherein:

FIG. 1 is a plot showing typical yield strength properties versus overaging time for 7075 aluminum alloys treated according to the present invention, and f FIG. 2 is a plot showing with respect to overaging time the resistance to failure of 7075 aluminum alloy treated according to the present invention while subject to stress and a corrosive environment.

The aluminum-base alloys to which the present invention is applicable are those of the so-called 7000 series, which contain zinc as their principal alloying element but which may contain other alloying elements (e.g. copper and magnesium) as Well. In particular, it is applicable to aluminum base alloys of the type containing copper in the range from about 1.2 to 2.0 weight percent, magnesium in the range from about 2.1 to 2.9 weight percent, zinc in the range from about 5.1 to 6.5 weight percent, and chromium in the range of 0.18 to 0.40 weight percent. An alloy with such a composition is commonly referred to as 7075 aluminum alloy, according to the numerical designation adopted by the aluminum industry. As applied to. these alloys, the invention results in greatly increased resistance to failure under conditions of stress and corrosion and the retention of at least 95 of the mechanical strength which they possess after the standard heat treatments known in the industry as the T6 and T651 temper treatments. The standard T6 temper heat treatment used in the aluminum industry for 7000 series alloys such as the 7075 and 7001 types consists in sequence of a solution heat treatment at a temperature of 850915 F. for a period long enough to allow the constituents of the alloy to enter into solid solution, a water quench, and artificial aging at a temperature of 230260 F. for a period of about 24 hours. The T651 treatment is essentially the same except that after the water quenching and before the artificial aging, the alloy is stretched /2 to 3% to eliminate stresses produced by the water quench and also, in the case of sheets, to make the alloy flat. A comparison of the design mechanical properties of 7075 alloy plate precipitation hardened to the T6 and T651 tempers is provided by Table I.

TABLE I Yield Ultimate strength Elongation in strength, 0.2% ofiset, two inches, Alloy and temper p.s.i. p.s.i. percent Although the T6 and T651 temper treatments develop mechanical strength, it is also known that each of these treatments has the effect of rendering the alloy susceptible to catastrophic failure when exposed to corrosive environments under stress.

For example, it has been found that short transverse tensile samples of aluminum alloy 7075 which have been heat-treated in the conventional manner to T651 temper tend to fail in about 30 minutes when loaded to 65,600 p.s.i. (approximately 90% of the yield strength of specimen) in 1 M NaCl solution buffered to a pH of 4.7 and corroded with an anodic current of 0.3 ma./cm. Attempted improvements in the stress-corrosion resistance of the alloy have included methods in which the alloy is aged at a higher-than-normal temperature, or overaged, or age hardened at a high temperature and overaged at lower temperature. In each instance, the result has seemingly improved the resistance of the alloy to stress corrosion, but to the extent this has been achieved there has been an accompanying and significant decrease in 0.2% yield strength. For example, when 7075 alloy is overaged to the T73 temper from the T6 temper, the 0.2% offset yield strength is reduced by about 9,000 p.s.i. This problem is overcome by the present invention.

Essentially the present invention involves solution heat treating specimens of the alloy to a temperature within the range of about 860 F. to 920 F., preferably about 900 F., for a period sufiicient to allow the alloying constituents to enter into solid solution. This period ranges from a few minutes to several hours, depending on the chemical composition and thickness of the specimens. Specifically, I have found that a soaking period of approximately 60-minute duration contributes favorably to the development of optimum properties. At the conclusion of the solution heat treating phase of the process, the specimens are then quenched at a rapid rate to a temperature of about 230260 F., preferably by immersion in an oil bath which is heated and maintained at this temperature range, preferably at about 250 F. Following quenching, the specimens are artifically aged to develop full mechanical properties, this latter treatment comprising maintaining the specimens at a temperature within the range of 230 F. to 260 F, preferably 250 F., for a period of 20-30 hours, preferably about 24 hours. This aging treatment may be accomplished by continued immersion of the articles in the quench medium, e.g., oil bath, providing the temperature of the quench medium can be maintained substantially constant. However, the quenched specimens can also be aged in an oven. At the conclusion of this aging step, the specimens are subjected to overaging by heating them to a temperature between 310 and 340 F., preferably about 325 F, for a period of not less than 7 and no more than about 20 hours. Preferably, to preserve optimum strength properties and to provide a high degree of resistance to stress-corrosion conditions, the articles are not cooled subsequent to the quenching and before age hardening and are overaged at about 325 F. for a period of at least 8 but not more than 18 hours, and more preferably about 12 hours.

The effect on mechanical strength and resistance to stress corrosion resulting from the process of the present invention is illustrated by the following example.

EXAMPLE Eight identical specimens of 7075T651 Alloy were prepared. These specimens were made in the conventional shape employed for strength tests, having two enlarged end sections with a /4 inch OD. and an intermediate section with an inch OD. and a length of V inch. The overall length of each sample 'was 1% inches. All of these specimens were solution heated to a temperature of 900 F. for a period of one hour. Thereafter, the specimens were immediately quenched in an oil bath maintained at a temperature of approximately 250 F. The quenched specimens were not removed from the oil bath which was maintained at a temperature of 250 F. The specimens were maintained at this temperature in the bath for a period of 24 hours to effect artificial aging. Two specimens (assigned numbers 1 and 2) were set aside for testing with no additional heat treatment. Thereafter, six specimens were placed in another oil bath which was maintained at a temperature of 325 F. Two specimens (assigned numbers 3 and 4) Were removed after four hours of overaging. Two other specimens (numbered 5 and 6) were removed from the bath after seven hours and forty-five minutes of overaging. The final two specimens (numbered 7 and 8) were removed from the bath after eighteen hours. Specimens l, 3, 5 and 7 were subjected to yield strength tests. Specimens 2, 4, 6 and 8 were subjected to corrosion resistance tests. The yield strength tests were conducted in accordance with established procedures for 0.2 percent offset yield stress tests. The corrosion resistance tests consisted of loading the specimens to of their respective 0.2 percent offset yield strengths in a 1 M NaCl solution buffered to a pH of 4.7 and corroded with an anodic current of 0.3 ma./cm. The solution Was circulated at a rate of 220 ml./min. and was maintained at a temperature of 30 C.

Table II illustrates the results of the yield strength and corrosion resistance tests.

TABLE II Yield Corrosion strength resistance time 0. 2% offset to failure (p.s.i.) (mi11.)

Specimen:

It is believed obvious that specimens 6 and 8 have corrosion resistances many times greater than those of specimens 2 and 4. Comparison of the corrosion resistances of these specimens with the corresponding characteristic of conventional 7075-T651 alloy indicates that the process of the present invention is capable of yielding more than a fold increase in stress corrosion resistance with only a minor, as low as 1%, decrease in yield strength.

A number of other tests conducted on specimens heat treated as above described but with the overaging treatment varied as to time, demonstrate that the overaging treatment initially has little effect on yield strength and seemingly produces a slight gain in strength. In this connection reference is had to FIG. 1 which is a plot of typical yield strength as a function of overaging time where the overaging is conducted at a temperature of 325 It is believed apparent that after approximately six hours of overaging treatment, the yield strength begins to decrease as a function of overaging time. Significantly, this tendency toward a decrease in yeild strength marks the beginning of a drastic improvement in the time to failure of the specimens when subjected to the stress corrosion tests. This is demonstrated by FIG. 2 which illustrates how the stress corrosion resistance, taken as the time to failure under the application of the stress load, varies with overaging time where the overaging is conducted at a temperature of 325 F.

As indicated by FIG. 2, it is to be noted that specimens artificially aged as above described, but not subjected to overaging treatment, will fail within an average of about 12 to 14 minutes. FIG. 2 clearly shows that there is no substantial increase in stress corrosion resistance if the alloy is overaged for about six hours or less. However, if the alloy is overaged for more than about six hours, the resistance to stress corrosion will improve markedly. The overaging treatment initially has little degrading defect and, as indicated by the yield strength curve of FIG. 1, seemingly even produces a slight gain in the alloys tensile strength properties. However, after approximately six hours of overaging at 325 F., the yield strength begins to decrease as a function of overaging time.

Considering the plots of FIGS. 1 and 2 together, it is believed apparent that overaging for a period between approximately 7 hours to approximately no more than 18 hours is necessary in order to achieve an optimum combination of stress corrosion resistance and yield strength. It is significant that if the specimens are overaged at 325 F. for a period of 18 hours, the yield strength decreases from approximately 73,000 p.s.i. which is characteristic of the conventional T651 temper to approximately 71,000 p.s.i., a loss or" only about 2.7%. On the other hand, the stress corrosion resistance increases by a factor of more than 100.

The significant improvement in stress corrosion resistance which is achieved is due to the unique combination of steps which make up the process of the invention. It has been determined that if the solutionized alloy is not quenched to a temperature in the neighborhood of 230- 260 F., the stress corrosion resistance and strength of the alloy will be no greater than that achieved with the conventional T73 temper treatment. Quenching in water at room temperature (as in the T6, T651 and T73 temper treatments), or even in warm water maintained at a temperature of 150 F. as mentioned in US. Pat. 3,231,435, will not provide the same results as quenching at the higher temperature utilized in the present invention. Furthermore, the use of an oil quench provides more uniform cooling and hence minimizes development of internal stresses. Quenching in oil heated to 250 F. provides a more homogeneous and less irregular heat transfer from the alloy specimens than does water quenching. As a result, the subsequent distribution of the precipitate particles which are formed after age hardening and overaging gives an improved combination of strength and resistance to stress corrosion. Furthermore, the alloy is more nearly free of internal stresses which unfavorably affect its stability. Deviation by as much as -15 degrees from the range of 230260 F. for the quench bath temperature is permissible and, depending on the composition and size of the alloy specimens, will not appreciably effect the strength properties of the heat treated alloy. Aging the alloy at a temperature appreciably lower than 230 F. will have the tendency of slowing up the aging process, with the result that maximum stress-corrosion resistance will not be attained as easily by the overaging treatment. Aging the alloy at a temperature appreciably above 260 F. tends to adversely affect the beneficial effect of the subsequent overaging treatment since the distribution of precipitates caused by the age hardening treatment tends to be excessively coarsened. The same coarsening tends to occur if overaging itself is conducted at a temperature appreciably above 340 F.

Although the example set forth above is concerned with 7075 originally in the T651 temper, the same improvement in stress corrosion resistance occurs if the alloy is originally in the T6 temper or any other heat-treated state. Similarly the invention is applicable to other aluminum alloys of the 7000 series as, for example, the 7001 alloy which characteristically includes 2.6 to 3.4 percent magnesium, 6:8 to 8.0 percent zinc, 1.6 to 2.6 percent copper, and 0.18 to 0.40 chromium. For the latter alloy and other alloys of the 7000 series, the temperature of the quench bath and the optimum tempera tures and times for the aging and overaging phases may be somewhat greater or less according to the specific composition of the alloy. However, the solutionizing temperature is essentially as herein prescribed. Except for these differences in operation conditions, the process is essentially the same, yielding a substantially material increase in stress corrosion resistance at the expense of only a slight decrease in yield strength. Other tensile properties, such as ultimate tensile strength and the percentage elongation over a given length, e.g., two inches of specimen, are modified to substantially the same extent as yield strength by this process. Hence on balance, aluminum alloys of the 7000 series are materially improved and their usefulness for structural applications is greatly enhanced by this invention.

A further important advantage of the invention is that it is applicable even after the alloy has been shaped, e.g., by rolling, into structural parts. Still another advantage is that it may be practiced successfully without conforming strictly to the specific procedure set forth above. Thus the specimens may be allowed to cool to room temperature after being quenched and before being subjected to artificial aging. It is also permissible to allow the alloy to cool to a temperature lower than the aging temperature before subjecting it to the overaging treatment. With any or all of these variations in procedure, the heat treated specimens will exhibit substantially the same mechanical properties and the same stress corrosion resistance as is achieved with the preferred procedure described above.

It is appreciated that other variations and modifications of the invention that do not depart from the principles herein set forth will be obvious to persons skilled in the art. Accordingly, it is to be understood that the matter contained in the foregoing description and shown in the accompanying drawings is to be interpreted in an illustrative rather than limiting sense and that the scope of the invention is to be determined solely from the appended claims construed in the light of this specification.

What is claimed is:

1. A process for heat treating a high-strength aluminum base alloy of the zincmagnesium-copper series in which the zinc content is greater than each of the magnesium and copper contents to reduce its susceptibility to stress-corrosion cracking while retaining its high strength properties comprising, heating the alloy to a temperature of between about 850 F. and about 915 F. for a period of time sufficient to allow the constituents of said alloy to enter into solid solution, quenching said heated alloy, aging the quenched alloy by heating it to a temperature between about 230 F. and about 260 F. for a period of between about 20 hours and 30 hours, and thereafter heating the aged alloy to a temperature between about 310 F. and 340 F. for a period of no less than about 7 and no more than about 20 hours, said alloy being quenched from the temperature of solution heat treatment into a liquid to a temperature of between about 230 F. and about 260 F.

2. The process of claim 1 wherein said alloy is quenched in an oil bath.

3. The process of claim 1 wherein said alloy is aged at a temperature of about 250 F. for a period of about 24 hours.

4. The process of claim 1 wherein said aged alloy is heated to a temperature of about 325 F. for a period of between about 7 hours and about 18 hours.

5. The process of claim 1 wherein said alloy comprises by weight about 1.2 to 2.0% copper, about 2.1 to 2.9% magnesium, and about 5.1 to 6.5% zinc and 0.18 to 0.40% chromium.

6. The process of claim 1 wherein said alloy comprises by weight 1.6 to 2.6% copper, 2.6 to 3.4% magnesium, 6.8 to 8.0% zinc, and 0.18 to 0.40% chromium.

8 7. A process for heat treating articles of aluminum overaging said articles at a temperature of between base alloys of the zinc-magnesium-copper series in which 310 F. to 340 F. for a period of at least 7 and no the zinc content is greater than each of the magnesium more than 20 hours. and copper contents so as to increase the resistance of the articles to failure when exposed to a corrosive medium while under conditions of high stress and also to References Cited UNITED STATES PATENTS develop high strength in said articles, said process com- 2,242,944 /1941 Dix et a1 148*159 2,695,253 11/1954 Schaaber 148 159 heatlng said articles to a temperature of between about 3 171 760 3/1965 Vernam et a1. 8 50" F. and about 915 F for a period sufiicrent to 10 3,231,435 1/1966 Rotsen et a1 dissolve the alloymg constituents into sohd solution; quenching said articles in oil heated to a temperature FOREIGN TENTS of between about 230 F. and about 260 R; 544,439 4 1942 Great Britain aging said articles in said oil at said temperature of between about 230 F. and about 260 F. for be- CHARLES N. LOVELL, Primary Examiner tween about hours and hours; and

" UNITED STATES PATENT OFFICE 56 CERTIFICATE OF CORRECTION Patent No. 3573117 Dated March 1971 Inventor) FRANKLIN H. COCKS It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected'as shown below:

Column 3, lines 6 and 7, the values "8" and "5" in Table I are changed to "5" and "8" respectively.

Signed and sealed this 22nd day of June 1971.

(SEAL) Attest:

LIIILLIAM SCHUYLER, JR.

1 I? I wwmn MELAGHBRJ commissioner f Patents Attesting Officer 

